This invention relates to a printing blanket, and more particularly to a printing blanket having a substantially smooth, nontextured, low abrasion base surface which contacts the cylinder on which the blanket is mounted. The type of blanket referred to herein is used primarily in offset lithographic printing, but may also find utility in other fields of printing.
In offset lithography, a rotary cylinder is covered with a printing plate which normally has a positive image area receptive to oil-based inks and repellent to water and a background area where the opposite is true. The printing plate is rotated so that its surface contacts a second cylinder covered with a rubber-surfaced ink-receptive printing blanket. The ink present on the image surface of the printing plate transfers, or offsets, to the surface of the blanket. Paper or other sheet stock to be printed is then passed between the blanket-covered cylinder and a rigid back-up cylinder to transfer the image from the surface of the blanket to the paper.
During the step in which the image is transferred from the plate to the blanket and the step where the image is transferred from the printing blanket to the paper, it is important to have intimate contact between the two contacting surfaces. This is ordinarily achieved by positioning the blanket-covered cylinder and the supporting cylinder it contacts so that there is a fixed interference between the two so that the blanket is compressed throughout the run to a fixed depth, typically approximately 0.002 to 0.004 inches. It is important that this compression be maintained uniformly over the entire surface of the blanket.
Conventionally, this fixed interference is accomplished by inserting one or more thin layers of paper or the like between the blanket and the stirface of the cylinder to build up the thickness of the blanket. This process is known as packing a blanket. This process presents problems however in that the packing procedure is time consuming, resulting in down time for the printing equipment. Further, once positioned on the cylinder, the packing paper tends to slide, slip, and/or fold which may render the blanket surface nonuniform and resulting in poor printing results. Further, when a blanket must be replaced, the time consuming packing operation must be repeated for a new blanket.
So-called "no pack" blankets have been developed to provide a fixed interference without the need to pack the blanket. No pack blankets are manufactured to very precise gauges so that they can be installed directly onto a cylinder with the correct amount of interference. These blankets have the advantage of a one-piece construction which requires no positioning of packing paper beneath the blanket. This results in less down time for the printing equipment when an old blanket is removed and replaced with a new blanket.
Such no pack blankets, like most printing blankets, are normally composed of a base material which gives the blanket dimensional stability. Woven fabrics are preferred. The base may consist of one or more layers of such fabric. The working surface of the blanket which contacts the ink is typically an elastomeric layer of natural or synthetic rubber which is applied over the base layer or layers. The base layer or layers and working surface are laminated together using suitable adhesives.
In offset lithography as well as other printing operation, the printing plate and blanket cylinders are subject to corrosion and rust because of exposure to inks, water, and chemicals used in cleaning up the machinery. To combat such problems, these cylinders have typically been plated with chrome or nickel. These metals provide a surface that is not only corrosion resistant, but also ink repellent.
However, such nickel- and chrome-plated cylinders have not worked well in conjunction with no pack blankets. After only short periods of use, the nickel plating is removed from the cylinder surface to such an extent that uncoated steel is exposed. While chrome plating is more resistant to removal than nickel, it too is subject to wear. The areas on the cylinder surface where the plated metal is removed are then subject to rapid corrosion and/or oxidation. Some have speculated that the nickel or chrome is removed by corrosion from chemicals which wick around the edges of the printing blanket. Others have speculated that the metal removal is caused by electrical charges building up from the friction between the blanket and cylinder.
Some attempts have been made to eliminate the removal of metal platings. For example, Sporing, U.S. Pat No. 4,471,011 provides a thin rubber coating on the base of a printing blanket to restrict the wicking of solvents and other chemicals beneath the blanket. Others have applied lubricants to the blanket cylinder to prevent such solvents and chemicals from contacting the blanket cylinder. Neither of these attempted solutions has done more than to slow down the removal of metal, and the application of lubricants must be repeated each time a blanket is changed.
In any event, it would be desirable to be able to use no pack blankets, with their attendant advantages, on such plated cylinders. Accordingly, the need exists in the art a no pack printing blanket which can be used successfully in conjunction with a nickel, chrome, or other corrosion resistant metal-plated cylinder.